Solar-stellar research and the dynamo problem

Abstract

Beginning in the late 1980s, helioseismology revealed the large scale rotation profile of the bulk of the solar interior. Almost at a stroke, entire classes of dynamo models -- designed to explain the 22 year solar magnetic cycle -- could be dismissed. Using the rotation data, kinematic models of the dynamo have met with some success, even demonstrating limited skill in forecasting properties of the solar cycle. However, more recent progress in numerical and observational studies have opened up new questions. Is the solar convection zone “essentially magnetized” in the sense that spatially intermittent but strong magnetic fields influence convection? What elementary process converts toroidal field, readily created by differential rotation, into poloidal field? What are the dominant modes of meridional circulation? Do flux transport models merely describe surface field evolution or do they genuinely contain essential physical processes at work? Do large – scale “wreaths” of magnetic field found in dynamical models exist purely within convection zones? How and why do the Sun and stars enter and exit global minima of their activity cycles (e.g., the Maunder Minimum)? These and longer standing questions, such as why the Sun/stars form spots at all, what drives the 11 year flip of global magnetic polarity, and what controls the diffusion of magnetism, require the acquisition and analysis of new and critical data. We argue that our understanding of the Sun and its enigmatic dynamo will be greatly complemented by more concerted comparisons with stars. Our purpose is to summarize the case for “re-running the solar-stellar experiment” through a modest but concerted line of funding to allow the community to continue to and expand on studies of carefully selected stars and stellar ensembles on long time scales. Multi-decade observations are essential, given the 22-year global magnetic activity cycle of the Sun. This cross-disciplinary activity is not new, but new reinvestment is timely. Results from KEPLER and other experiments have revealed accurate critical parameters for many Sun-like stars. Spectroscopy and photometry reflecting magnetically-induced variations now span several decades for the Sun and several well-observed stars. Continued and expanded observation programs will have broader impacts in stellar rotational evolution, the solar-terrestrial connection, space weather, and astrobiology, providing important constraints for diverse theoretical models.